FREE RADICAL GRAFTING OF MONOMERS ONTO POLYPROPYLENE RESINS

Abstract: A method of free radical grafting of monomers onto a polypropylene resin, comprising the steps of: (i) mixing the polypropylene resin with an allylepoxy compound, a styrenic compound and a peroxide as an initiator, (ii) heating said mixture to a temperature which is higher than the melting point of the polypropylene resin and the decomposition temperature of said peroxide, at conditions of continuous mixing until a desired degree of grafting of the allylepoxy compound and the styrenic compound onto the polypropylene resin has been achieved, and (iii) cooling and comminuting the obtained composition. A grafted polypropylene composition being obtained by said method comprises an amount of glycidyl methacrylate of 0 to 10 % by weight and an amount of styrene of 0 to 10 % by weight, calculated on the polypropylene resin. The composition may be used as a compatibilizer in the manufacturing of polymer blends, as a polymer adhesion promotor, and in articles which require an improved printability.

Description: Free radical grafting of monomers onto polypropylene resins Technical field. This invention relates to a method of free radical grafting of monomers onto polypropylene resins. More particu¬ lar the invention relates to a method of the simultaneous grafting of two different monomers, preferably an allylepoxy compound and a styrenic compound, onto a polypropylene poly¬ mer, and the material obtained in said method. Background art The modification of polypropylene resins by grafting specific compounds onto the polymer backbone chain in order to produce compositions having improved properties, is previously know . EP 0.280.454 discloses a method for the grafting of at least two different monomers onto molten hydrocarbon poly¬ mer. In working examples the polymer is a homopolymer of pro- pylene, a high density polyethylene or linear low density polyethylene, and the monomers are styrene or maleic anhydride. Contrary to the present method, the process is operated in the absence of an initiator for the grafting reac¬ tion and in the substantial absence of antioxidant in the polymer, by the use of an extruder at extrusion conditions. US 5.079.295 relates to a thermoplastic composition comprising a polyphenylene ether resin, a modified propylene polymer and a rubbery substance. The polypropylene component is grafted with a styrene-based monomer and an unsaturated carboxylic acid or its derivative. A number of grafting pro- cesses are described. In the working examples 100 parts by weight of polypropylene are mixed with one part of maleic an¬ hydride (MAH), one part of styrene and one part of peroxide in a mixer, and subsequently extruded at 220 β C. In another embodiment in an autoclave a dispersion of polypropylene, styrene and MAH in water is allowed to react at 120°C. In further embodiments the MAH of said dispersion is replaced by glycidyl methacrylate or glycidyl acrylate. NO patent application no. 924786 relates to poly¬ propylene compositions grafted with glycidyl acrylate or gly- cidyl methacrylate (GMA) using an organic peroxide as a radi¬ cal-generating agent, performed by kneading the melted mix¬ ture, preferably in an inert atmosphere. In a grafting process, the high molecular weight of 5 the virgin polypropylene resin should be maintained to retain its desired physical and mechanical properties. If only one type of monomer, such as a monomeric allylepoxy compound, is used in a polypropylene grafting method, the achieved grafting efficiency is poor and accompanied by a heavy degradation of 0 the polymer by chain scission in the β-position and simultane¬ ous homopolymerization of the added monomer. Such a degrada¬ tion of the polypropylene will inevitably result in polypropy¬ lene compositions having a reduced molecular weight, and con¬ sequently articles manufactured therefrom will have mechanical s properties inferior to those of articles made from virgin polypropylene resins. The problem of chain scission would be avoided if the grafting could be performed in the presence of a compound having a stabilizing effect on the intermediate polymeric radicals created during the grafting process. The o composition would then maintain its high molecular weight and its highly valued physical properties. Summary of the invention It has now surprisingly been found that the degrada- 5 tion of the polypropylene polymer by β-scission during a free radical initiated grafting of monomers onto the polypropylene polymer chains can be strongly limited when said grafting is performed in the presence of a styrenic compound. Thus, a polypropylene resin in a molten state may be grafted with a o monomeric allylepoxy compound in the presence of a styrenic compound without any noticeable degradation of the poly¬ propylene polymer itself. The present invention thus provides a method of free radical grafting of monomers onto a polypropylene resin having 5 a melt flow index of 0.2 to 100 g/10 minutes, preferably 0.2 to 50 g/10 minutes (measured according to ASTM D 1238 at 230°C/2.16 kg), comprising the steps of: (i) mixing the polypropylene resin with an allylepoxy compound having the formula: in which R is H or a C x . 4 alkyl; and R λ is -(CH 2 ) n -; -C(0)0-(CH 2 ) n -; or -(CH 2 ) n -0-; and n is an integer of 1 to 4; a styrenic compound having the formula: - R 2 in which R 2 is H, OH, CH 3 or allyl; and a peroxide as an initiator, (ii) heating said mixture to a temperature which is higher than the melting point of the polypropylene resin and the decomposition temperature of said peroxide at conditions of continuous mixing until a desired degree of grafting of the allylepoxy compound and the styrenic compound onto the poly¬ propylene resin has been achieved, and (iii) cooling and comminuting the obtained com- position. The invention also provides a grafted polypropylene composition being obtained by the method described above. The composition of the present invention, may be used as a compatibilizer in the manufacturing of a polymer blend, as a polymer adhesion promotor, and in articles which require an improved printability. Detailed description of the invention It is an object of the present invention to achieve a method of free radical grafting of monomers onto a polypropy¬ lene resin, which method being performed at conditions resul¬ ting in an improved level of grafting without any substantial degree of polypropylene degradation by chain scission. This is accomplished when the grafting of an allylepoxy compound is performed in the presence of a styrenic compound. The polypropylene resins that may be used in the grafting method of the present invention, comprise polypropy¬ lene homopolymers and copolymers of propylene with ethylene and/or butadiene, in particular resins having a weight average molecular weight of 150,000 to 500,000 and a melt flow index of 0.2 to 100 g/10 minutes, preferably 0.2 to 50 g/10 minutes, determined at 230°C and 2.16 kg load according to the method of ASTM D 1238. Copolymers may contain up to 20 % by weight, preferably 2 to 12 % by weight, of ethylene. The monomeric allylepoxy compound to be grafted onto the polymer chains must contain polar or" functional substi- tuents. Said monomers are preferably chosen from the group comprising allylepoxy compounds having the formula: in which R is H or a C^ alkyl and ^ is -(CH 2 ) n -; -C(0)0-(CH 2 ) n -; or -(CH 2 ) n -0-; and n is an integer of 1 to 4. Preferably R is H or CH 3 , and more preferably CH 3 . R α is prefe¬ rably -C(0)0-(CH 2 ) n -. Thus the most preferred compound is gly¬ cidyl methacrylate. Suitable styrenic compounds are those having the for- mula: in which or allyl. Preferably R 2 is H, making styrene the most preferred styrenic compound. The styrenic compound is assumed to stabilize by delocalization the free radical species being present during the grafting process. Other compounds than those mentioned above having conjugated unsaturated double bonds, e.g. quinone, may give similar results. To initiate the grafting process any free radical generating peroxide compound known in the art having a suit¬ able decomposition temperature, may be used. In particular the following peroxide initiators have been found to give accep¬ table results: 2,5-bis(tert-butylperoxy)-2,5-dimetylhexane ("DHBP"), bis(tert-butylperoxyisopropyl)benzene ("Perkadox- 14") and 2,5-bis(tert-butylperoxy)-2,5-dimethyl-3-hexyne ("Trigonox-145"). Intimate mixing of the polypropylene resins, the initiator and the grafting monomers at a temperature higher than both the melting point of the polypropylene resin and the peroxide decomposition temperature is a prerequisite for achieveing a desired degree of grafting in the present method. The required grafting conditions can be accomplished both in a s batch process and a continuous process. In the batchwise grafting process any suitable, closed batch mixer of a conventional type equipped with a con¬ tinuous stirring device can be used, for example a Haake mixer. The polypropylene resin is introduced along with the styrenic compound, the allylepoxy compound and the initiator into the preheated mixer operated at a mixing speed of 10 to 200 rpm and at a temperature of 150 to 200 β C. Normally an acceptable degree of grafting is accomplished within a time s period of less than 15 minutes at said temperatures. Preferably, the grafting is performed in a continuous manner, and an extruder may be used for the purpose. The graf¬ ting conditions are adapted to the type of extruder used, as will be well known to a man skilled in the art. The most o important features of the process are to secure adequate mixing and a uniform temperature of the polymer melt, as well as a sufficient dwell time to ensure the completion of the grafting reactions. A double screw extruder having co-rotating intermeshing screws is preferably used, for example a ZSK 5 Werner & Pfleiderer machine. In both the batchwise and the continuous grafting method the components may be fed to the mixing machine in any practical way. The sequence of adding the components is not critical. In the continuous extrusion method the components o may optionally be premixed, which may be done by the use of any conventional mixer and mixing method known in the art. To obtain an optimal degree of grafting, 100 parts by weight of polypropylene resin is mixed with up to 10 parts by weight of the styrenic compound and up to 10 parts by weight 5 of the allylepoxy compound. Obviously the actual weights of the styrenic and the allylepoxy compounds will depend on the specific types of compounds used, and the invention is not restricted to the use of specific amounts of the grafting com¬ pounds. The initiator is usually added in an amount of up to 0.25 % by weight of the starting polypropylene resin, but higher amounts may also be used. A problem connected with the grafting method is a possible contamination of the reagents with oxygen from the 5 surrounding air. Oxygen will react spontanously with the gene¬ rated radicals, giving oxy-radicals which are particularly active agents in polymer chain scissions; Therefore, the graf¬ ting method is preferably carried out under an inert atmos¬ phere, most preferably in an atmosphere of nitrogen. ° The polymer chain scissions and the molecular weights of the present compositions can be controlled by the use of the comonomer system described herein. In the grafting re¬ actions taking place the styrenic compound will function as a chain transfer agent. When the grafting is performed at the s conditions of operation described herein, the degradation of the polypropylene backbone chain by β-scission will be sig¬ nificantly lower than what can be achieved in traditional gra¬ fting processes without the use of a chain transfer compound. In an embodyment of the present invention as demon- o strated by the working examples, at a given constant concen¬ tration of glycidyl methacrylate the grafting efficiency of glycidyl methacrylate increases when the styrene concentration increases, while the incorporation efficiency of styrene is substantially constant. When styrene monomer being present, 5 the glycidyl methacrylate grafting level increases more than two times compared with the case of styrene not being present. A composition obtained by the present method will comprise an amount of glycidyl methacrylate of 0 to 10% by weight, preferably 2 to 5% by weight, and an amount of styrene 0 of 0 to 10% by weight, preferably 2 to 5% by weight, said weights being calculated on the basis of the starting, neat polypropylene resin. The styrenic compound also acts as a comonomer, reac¬ ting with the allylepoxy monomers giving random copolymers. 5 Thus, the side chains grafted onto the polypropylene backbone are random copolymers of said two species. The bulky benzene groups of said copolymers will exert a sterical effect making the reactive sites more accessible to the polymerizing mono¬ mers of the reaction mixture, and as a consequence a higher amount of glycidyl methacrylate can be copolymerized and graf¬ ted onto the polypropylene resin. The increased amount of incorporated glycidyl methacrylate increases the number of polar groups in the polypropylene composition, enhancing its compatibilizing effect. The styrenic compound has a combina¬ torial effect in that it is both participating as a comonomer in the copolymerization reactions and it is simultaneously acting as a chain transfer compound in said reactions. The polymeric composition obtained in the grafting method of the present invention will have a higher amount of allylepoxy compound and styrenic compound grafted onto the polymer chain than what is obtainable by any other known process, while the extent of polypropylene chain scissions is substantially reduced compared to other grafting processes. The composition obtained in the method of the present invention is particularly suitable as a compatibilizing pre¬ cursor in polymer blends of various types. Typical amounts to be used in such blends are within the range of 0.1 to 15 % by weight, preferably 1 to 5 % by weight, based on the weight of the final blend. The grafted polypropylene composition of the present invention may also be used as a polymer adhesion promotor, in bonding layers, and being utilized in coating processes. More¬ over, the increased polarity of the grafted polypropylene com¬ position will improve the paintability of articles manufactured therefrom. The invention will now be illustrated and described in more detail by ways of examples, which have not be con¬ strued as limiting the invention. F.vamp1*»« Batch grafting method. Polypropylene resin, styrenic monomer, glycidyl meth¬ acrylate (GMA) and peroxide, are introduced into a conventio¬ nal closed Haake mixer equipped with a stirring device. The mixer is running at a constant speed of 64 rp during the entire grafting process. Samples of hot, grafted polymer are withdrawn and cooled in air of ambient temperature or in liquid nitrogen. s Continuous grafting method. Polypropylene compositions are grafted continuously in a ZSK Werner & Pfleiderer double screw extruder with corotating intermeshing screws of length 1230 mm and L/D = 42. The screw rotating speed is 150 rpm in all the presented exam- o pies. The polypropylene material premixed with styrenic, GMA and initiator, is fed to the extruder at a constant rate of 3.5 kg/h of polypropylene material. The present temperature profile in the extruder is: 5 Compres¬ Metering Barrel zones: Feeding sion Mixing and die Temperature ( °C) 200 200 230 240 0 The polypropylene polymer is extruded as strands, cooled by water and then pelletized. Analysis of grafted polypropylene compositions. 5 Collected samples of polypropylene compositions are dissolved in hot xylene, then precipitated by adding large amounts of acetone of ambient temperature to the hot solution. The precipitate is dried under vacuum at 50 β C for 8 hours. The amounts of grafted styrene and/or GMA onto the polypropylene 0 resins are determined by IR spectrometry. The weight and num¬ ber average molecular weights, „ and M„, respectively, of each sampled polymer are measured by size exclusion chromatography (SEC). 5 Experimental data and achieved results are indicated in tables 1 to 4. The concentrations of styrene, glycidyl methacrylate and peroxide in the starting reaction mixture are denoted [styrene]^ [GMA]^ and [peroxide] , respectively, while the amounts of styrene and glycidyl methacrylate grafted onto the polypropylene resins are denoted [styrene] g and [GMA] g , respectively. The grafting efficiency of GMA and styrene given in the tables indicate the amount of each com¬ pound grafted onto the polymer resin in percent of the total 5 amount of said compound introduced into the starting reaction mixture. The presented molecular weight distribution, MWD, is calculated as: MWD = M^ -,. In the following examples all quantities given in percent by weight are based on the weight of the ungrafted 0 polypropylene polymer starting resin, unless otherwise stated. Grafting experiments in a discontinuous Haake mixer. s Comparative example A Polypropylene resin in the form of powder is intro¬ duced into the mixer stirred at 64 rpm and heated to 200 β C. After 15 minutes of reactive mixing at 200°C, hot samples are collected and cooled in air of ambient temperature. The measu- 0 red molecular weight and the calculated molecular weight di¬ stribution are presented in Table 1. Comparative example B Polypropylene powder premixed at ambient temperature 5 with 0.22 % by weight of "DHBP" peroxide, is introduced into the mixer. The mixing and sampling conditions are as in com¬ parative example A. The obtained molecular weight and molecu¬ lar weight distribution of the sample are presented in Table 1. 0 Comparative examples C, D. E and F Polypropylene powder premixed at ambient temperature with 0.22% by weight of "DHBP" peroxide and 0.58, 1.49, 4.44 and 7.48% by weight, respectively, of styrene, is introduced 5 into the mixer. The mixing and sampling conditions are as in comparative example A. The amounts of grafted styrene of the polypropylene compositions are presented in Table 1. Comparative example G Polypropylene powder premixed at ambient temperature with 0.24% by weight of "DHBP" peroxide and 5.84% by weight of GMA monomer, is introduced into the mixer. The mixing and sam- s pling conditions are as in comparative example A. The obtained molecular weight, the molecular weight distribution and the amount of grafted GMA of the polypropylene composition are presented in Table 2 below. o Examples 1 - 4 Polypropylene powder premixed at ambient temperature with 0.24% by weight of "DHBP" peroxide, between 5.8 and 6.1% by weight of GMA, and 2.18, 3.50, 4.40 and 6.53% by weight re¬ spectively of monomeric styrene is introduced into the mixer. s Hence, the ratio of [styrene] A to [GMA^ varies from 0.5 to 1.5 on a molar basis. The mixing and sampling conditions are as in comparative example G. The results obtained are presented in Table 2. 0 Comparative example H Polypropylene powder premixed at ambient temperature with 5.82% by weight of GMA and 8.73% by weight of styrene are introduced into the mixer. Mixing and sampling conditions are as in comparative example G. The obtained molecular weight, 5 molecular weight distribution, amounts of grafted GMA and sty¬ rene of the polypropylene composition are presented in Table 3 below. Examples 5 - 8 0 Polypropylene powder premixed at ambient temperature with 5.82% by weight of GMA, 8.73% by weight of styrenic and from 0 to 0.44% by weight of "Perkadox-14" peroxide is intro¬ duced into the mixer. Mixing and sampling conditions are as in comparative example H. The obtained molecular weights, olecu- 5 lar weight distributions, and amounts of grafted GMA and styrene of the obtained polypropylene compositions are presen¬ ted in Table 3 below. Continuous grafting at extrusion conditions. Examples 9 and 10 Polypropylene powder premixed at ambient temperature s with 0.30% by weight of "Perkadox-14" peroxide and with GMA and monomeric styrene in amounts as indicated in table 4 below, keeping a constant ratio of [styrene ] L / [GMA] 1 - 1.40 on a molar basis, is fed to the extruder. The extrusion condi¬ tions are as given above. Samples are collected at the extru- o sion die and the amounts of grafted GMA and styrene in the obtained compositions determined. The obtained results are presented in Table 4. 5 0 5 0 5 Table 3 [Peroxide] x [GMA], [Styrene], Graft eff. Graft eff. GMA Styrene MWD Example % wt % wt % % g/mol Comp. H 0 0.06 0 1.0 366,000 3.1 5 0.05 1.16 0.65 19.9 7.4 259,000 3.0 6 0.11 1.30 0.78 22.3 8.9 297,000 3.2 7 0.20 1.65 1.07 28.3 12.3 250,000 3.1 8 0.44 2.02 1.50 34.7 17.2 235,000 1.36 Table 4 [Peroxide]i [GMA] t [Styrene] t [GMA], [Styrene], Graft eff. Graft eff. Example GMA Styrene % wt % wt % wt % wt % wt % % 9 0.30 1.40 1.96 0.52 0.39 37.1 19.9 10 0.30 3.00 4.20 0.92 0.69 30.7 16.4

Claims: C l a i m s 1. A method of free radical grafting of monomers onto a polypropylene resin having a melt flow index of 0.2 to 100 g/10 minutes (230°C/2.16 kg), c h a r a c t e r i z e d i n comprising the steps of: (i) mixing the polypropylene resin with an allylepoxy compound having the formula in which R is H or a C^ alkyl; ^ is -(CH 2 ) n -; -C(0)0-(CH 2 ) r or -(CH 2 ) n -0-; and n is an integer of 1 to 4; a styrenic compound having the formula: in which R 2 H, OH, CH 3 or allyl; and a peroxide as an initiator, (ii) heating said mixture to a temperature which is higher than the melting point of the polypropylene resin and the decomposition temperature of said peroxide, at conditions of continuous mixing until a desired degree of grafting of the allylepoxy compound and the styrenic compound onto the poly¬ propylene resin has been achieved, and (iii) cooling and comminuting the obtained com¬ position. 2. The method of claim 1, c h a r a c t e r i z e d i n that the melting and mixing of the reagents is performed in an inert atmosphere, preferably in an atmosphere of nitrogen. 3. The method of claims 1 and 2, c h a r a c t e r i z e d i n that the grafting process is performed in a closed mixer at conditions of continuous agita¬ tion. 4. The method of claims 1 or 2, c h a r a c t e r i z e d i n that the grafting process is performed in an extruder. s 5. The method of claims 1 to 4, c h a r a c t e r i z e d i n that the grafting is performed at conditions where the styrenic compound both acts as a chain transfer compound and as a copolymerizable monomer, whereby the β-scission of the polypropylene polymer being lower than o what is obtainable with neat polypropylene resin being processed alone. 6. The method of claims 1 to 5, s c h a r a c t e r i z e d i n that the allylepoxy compound is glycidyl methacrylate. 7. The method of claims 1 to 6, c h a r a c t e r i z e d i n that the styrenic compound is o styrene. 8. A grafted polypropylene composition, c h a r a c t e r i z e d b y being obtained by the methods of claims 1 to 7. 5 9. The composition of claim 8, c h a r a c t e r i z e d i n comprising an amount of glycidyl methacrylate of 0 to 10% by weight and an amount of styrene of 0 to 10% by weight, calculated on the polypropylene 0 resin. 10. A use of the composition obtained by the methods of claims 1 to 9, as a compatibilizer in the manufacturing of a polymer blend, as a polymer adhesion promotor, and in articles 5 which require an improved printability.

Inventors: HU GUO HUA, KOTLAR HANS KRISTIAN, LAMBLA MORAND, SUN YI JUN, HU, GUO, HUA, KOTLAR, HANS, KRISTIAN, LAMBLA, MORAND, SUN, YI, JUN
Applicants: BOREALIS AS, HU GUO HUA, KOTLAR HANS KRISTIAN, LAMBLA MORAND, SUN YI JUN, BOREALIS A/S, HU, GUO, HUA, KOTLAR, HANS, KRISTIAN, LAMBLA, MORAND, SUN, YI, JUN
Application year: 1994
Main classification: C08F255
IPC codes: C08F255, C08L51